Energy in Daily Life

Look at people digging a field, carrying a heavy doko up a slope, or running a race. What does their body need for this? It needs energy. And what runs a bus, a plane or a factory machine? Energy again, from diesel, petrol, kerosene, coal or electricity.

Energy is the capacity (ability) to do work. Anything that can push, pull, lift, heat, light up or make a sound has energy. We measure energy in joules (J).

We get our energy from the food we eat. Food comes from plants and animals, and plants make food using sunlight. So the energy in your dal bhat first came from the sun.

Renewable and non-renewable sources of energy

What is used to cook food in your home? Most homes in Nepal use one of these: firewood, biogas (gobar gas), LP gas (liquefied petroleum gas, the gas in the red or blue cylinder), or an electric heater or induction stove. Where do they come from?

• Firewood comes from trees in forests.
• Electricity in Nepal comes mostly from the energy of flowing water: this is hydroelectricity (“hydro” means water).
• LP gas is separated out when crude oil from deep under the ground is refined (cleaned and split into parts). Nepal buys all of its LP gas, petrol, diesel and kerosene from other countries.
• Biogas is made from cow dung that rots in a closed tank.

We also need energy to run vehicles, to keep warm in winter, to dry grain, and to run tools and machines at a building site.

A substance or thing from which we get energy is called a source of energy. Firewood, biogas, coal, LP gas, petrol and diesel, flowing water, wind and the sun are all sources of energy.

Some sources are made again by nature in a short time after we use them. These are renewable sources of energy. Sunlight comes back every morning; rivers keep flowing because rain and snow keep filling them; cattle make fresh dung every day; new trees can grow in place of the ones we cut. Solar energy, wind energy, hydroelectricity, biogas (gobar gas) and firewood are renewable. Because they don’t run out, we should use them as much as we can.

Other sources take millions of years to form. Once we use them up, nature can’t make more in our lifetime. These are non-renewable sources of energy. The more we use them, the less is left, and one day they may run out. Coal, petrol, diesel, kerosene, LP gas and natural gas are non-renewable. So is nuclear energy, which comes from a rare metal called uranium, dug from mines.

Try sorting some sources yourself.

Fossil fuel

Most vehicles today run on diesel or petrol. Trains and factory machines can be run by the heat from burning coal. Have you ever wondered how petrol, diesel, kerosene and coal were made?

Long, long ago, great swampy forests grew on the land, and the seas were full of tiny living things (plankton, smaller than a grain of sand). When they died, they were buried under thick layers of mud and sand. Changes in the Earth’s surface pushed these layers deeper and deeper. Over millions of years, the great pressure (squeezing) and heat deep underground slowly changed them:

• the buried plants of the swamp forests became coal;
• the buried tiny sea creatures and plants became crude oil (mineral oil, also called petroleum) and natural gas.

The remains of living things from long ago are called fossils. So fuels made from them, coal, crude oil and natural gas, are called fossil fuels. Petrol, diesel, kerosene and LP gas are all made from crude oil, so they are fossil fuels too.

Advantages of fossil fuels

1. Fossil fuels are easy to get compared with many other sources: there are petrol pumps and gas depots everywhere.
2. They can be carried easily from one place to another, in tankers, pipes and cylinders.
3. They have many uses. They run vehicles, generators and factories, and cook food.
4. They give a very large amount of heat when burnt, so industries and factories use them a lot.

Disadvantages of fossil fuels

1. We are using them up fast, and nature can’t make more in time. One day they may run out.
2. Burning them pollutes the environment. The smoke has harmful gases and soot that make the air dirty and cause breathing diseases, and the carbon dioxide they give off traps heat and warms the whole Earth.

Alternative energy

A source of energy that we use instead of non-renewable fuels, to use less of them, is called alternative energy. In Nepal, the main alternative sources are hydroelectricity, solar energy and gobar gas (biogas).

• Hydroelectricity. Nepal’s rivers rush down from the Himalaya. Their water turns turbines, which turn generators that make electricity. Almost all of Nepal’s electricity is hydroelectricity, and in the rainy season Nepal even sells some to India.
• Gobar gas (biogas). Nepal is an agricultural country. Many families keep cattle for farming, milk and ghee. Their dung (gobar) is mixed with water in a closed underground tank. Without air, tiny living things (bacteria) break it down and give off a gas that burns with a clean blue flame: biogas, used for cooking. What is left is good manure for the fields.
• Solar energy. In remote, hilly places where power lines are hard to bring, a solar panel can give a home electricity for lights, a radio and a phone charger. Solar water heaters on roofs in towns heat water for bathing.

For Nepal’s land and way of life, solar energy and gobar gas are some of the best alternative sources for villages, and hydroelectricity for the whole country. To help people use them, the Government of Nepal gives a subsidy (pays part of the cost) for building biogas plants and putting up solar panels.

Biofuel and geothermal energy

Fuels made from things that were alive recently, not millions of years ago, are called biofuels. Firewood, dried dung cakes, biogas and oil pressed from plant seeds are biofuels. They are renewable, because new plants grow and animals make new dung.

Deep inside the Earth it is very hot. In some places this heat comes up to the surface and warms the underground water. The heat energy from inside the Earth is called geothermal energy (“geo” means Earth, “thermal” means heat). Nepal has many hot springs, called tatopani, for example in Myagdi and Sindhupalchok, where people bathe in naturally hot water. Countries such as Iceland use geothermal heat to make electricity and to warm their houses. It is a renewable source.

Ways of saving energy

All over the world, people use more and more fossil fuel every year. If we keep using non-renewable sources this fast, there will not be enough energy for everyone in the near future. A shortage of energy, caused by using too much non-renewable energy, is called an energy crisis.

To avoid an energy crisis, we must conserve (save) energy. Using energy carefully and wisely, without wasting it, is called conservation of energy.

These steps can reduce the energy crisis in Nepal:

1. Avoid using energy when it isn’t needed: switch off lights, fans and the TV when you leave a room.
2. Nepal is an agricultural country: build more gobar gas plants, especially in remote villages.
3. Make more hydroelectricity, and use it to cook food, run electric vehicles and do other work.
4. Use electricity from solar panels for many purposes.
5. Use alternative sources of energy instead of crude oil, coal and natural gas.
6. Run public awareness programs so that people learn the ways to save energy.

What do we need to dry grain and clothes, and to cook food? We need heat. Where does it come from?

The sun is our main source of heat. Its heat dries clothes, firewood and grain spread on a mat; it warms us in winter; and it dries sea water to leave behind salt. We also get heat from burning firewood and fossil fuels, and from electric heaters.

Heat is a form of energy that gives us the feeling of warmth. Like all energy, heat is measured in joules (J), the SI unit of heat. When we give heat to an object, its temperature rises.

Temperature

Three glasses stand on a table. One has lukewarm water, one has tap water, and one has very cold water. Can you tell which is which just by touching them? Easily.

Now try three glasses of lukewarm water, warmer water, and water that was boiled a little while ago. All three feel hot. It is hard to put them in order by touch, but their degree of hotness is different.

The degree of hotness or coldness of an object is called its temperature. Water with a higher degree of hotness has a higher temperature.

What makes hot water hot? Water, like everything, is made of tiny molecules, far too small to see. They are never still: they jiggle and move about all the time. In hot water they move faster; in cold water they move more slowly. The temperature tells us how fast, on average, the molecules are moving.

Thermometer

Have you had your temperature taken when you had a fever? The instrument used to measure the degree of hotness or coldness of an object is called a thermometer. There are different kinds:

• A clinical thermometer measures the temperature of the human body. It reads only from about 35 °C to 42 °C, because a living person’s body is never much hotter or colder than that. A thin kink just above its bulb stops the liquid from falling back, so the doctor can read it after taking it out of your mouth.
• A digital thermometer shows the temperature as numbers on a small screen. Today people use it instead of the glass clinical thermometer, because it is safer (no glass to break, no poisonous mercury) and quicker and easier to read accurately.
• A laboratory thermometer measures the temperature of other substances, such as water, ice or a solution, in a laboratory. It usually reads from −10 °C to 110 °C.

Laboratory thermometer

A laboratory thermometer is a long, thin cylinder of glass.

• At one end is a bulb with a thin glass wall, so that heat passes into it quickly.
• The bulb is joined to the stem. Through the middle of the stem runs a very narrow hole, finer than a hair, called the capillary tube. It goes from the bulb right to the other end of the stem.
• Around the capillary tube is thick glass, which makes the thermometer strong and acts like a magnifying glass, so the thin thread of liquid looks wider.
• The bulb is filled with mercury (a silvery liquid metal) or alcohol (colored red or blue).
• A scale in degrees Celsius is marked on the stem.

How does it work? To measure the temperature of a substance, we put the bulb in it, or touch the bulb to it. If the substance is hotter than the thermometer, heat flows into the bulb and the liquid expands (takes up more space). It has nowhere to go except up the capillary tube, so the level rises. If the substance is colder, the liquid contracts (takes up less space) and the level falls. Where the level stops, against the scale, gives the temperature.

Because the tube is so narrow, a tiny change in the volume of the liquid makes a big change in its height. That is what makes a thermometer able to show small changes in temperature.

Units of temperature

What is the temperature around you right now? Guess, then check it on a thermometer or a weather app.

When you have a fever, your temperature is measured to see how high it is. In Nepal, body temperature is usually measured in degrees Fahrenheit (°F). The normal body temperature of a healthy person is about 98.6 °F. During a fever, it rises above this.

Temperature is also measured in degrees Celsius (°C), the unit used in weather reports and in science classes. Although °C and °F are both widely used, the SI unit of temperature is the kelvin (K).

Relationship between different scales of temperature

Heat can change the state of a substance: it turns a solid into a liquid, and a liquid into a gas.

• The fixed temperature at which a solid turns into a liquid is its melting point. At sea level, ice melts at 0 °C: the melting point of ice is 0 °C.
• The fixed temperature at which a liquid turns into a gas (boils) is its boiling point. At sea level, water boils at 100 °C: the boiling point of water is 100 °C.

These two temperatures are easy to make anywhere, so they are used to mark every thermometer. They are called the fixed points. Here they are on the three scales, Celsius, Fahrenheit and Kelvin:

Scale	Celsius	Fahrenheit	Kelvin
Melting point of ice (lower fixed point)	0 °C	32 °F	273 K
Boiling point of water (upper fixed point)	100 °C	212 °F	373 K

Between the two fixed points, the Celsius scale has 100 equal steps (100 − 0), the Fahrenheit scale has 180 (212 − 32), and the Kelvin scale has 100 (373 − 273). So a temperature that is, say, halfway between melting ice and boiling water is halfway along every scale. This gives the formula that changes a temperature from one scale to another:

$$\frac{C - 0}{100 - 0} = \frac{F - 32}{212 - 32} = \frac{K - 273}{373 - 273}$$

which is the same as

$$\frac{C - 0}{100} = \frac{F - 32}{180} = \frac{K - 273}{100}$$

Here C, F and K are the same temperature written on the three scales. To change Celsius into Fahrenheit, use the first two parts: $\frac{C - 0}{100} = \frac{F - 32}{180}$. To change between Celsius and kelvin, use the first and the last.

Mathematical problems

The Celsius and Kelvin scales both have 100 steps between the fixed points, so one kelvin is the same size as one degree Celsius. That gives two handy shortcuts:

• kelvin = Celsius + 273, and Celsius = kelvin − 273;
• Fahrenheit = Celsius × 9 ÷ 5 + 32 (because 180 ÷ 100 = 9 ÷ 5).

Relationship between heat and temperature

Why does a cup of hot tea get cold after a while? Why does tap water feel so cold in winter?

Hot tea is at a higher temperature than the air around it. While it was boiling on the stove, it took in heat. On the table, it gives heat away to the cooler air and the cup, so the heat in the tea goes down and its temperature falls. It keeps cooling until it reaches the temperature of the room.

This shows the most important rule about heat:

Heat always flows from a hotter object (higher temperature) to a colder object (lower temperature), never the other way by itself. An object’s temperature rises when it gains heat and falls when it loses heat.

In winter, the air is cold, and the water in the pipes and wells cools down to that low temperature. Your body stays at about 37 °C. When you touch the cold water, heat flows from your hand into the water, and your skin loses heat quickly. That loss of heat is what you feel as “cold”.

Heat	Temperature
A form of energy	The degree of hotness or coldness
Flows from a hotter object to a colder one	Tells us which way heat will flow
SI unit: joule (J)	SI unit: kelvin (K)
Measured with a calorimeter (as you will learn in higher classes)	Measured with a thermometer
Depends on the amount of the substance (more water holds more heat)	Does not depend on the amount: a cup and a bucket of boiling water are both at 100 °C

Energy can get from one place to another in two ways.

1. Something moves and carries the energy with it. A bullet fired from a gun, or a stone thrown at a mango tree, carries its energy all the way.
2. The energy travels, but the substance stays where it is. This happens in a wave.

Throw a stone into the middle of a still pond. The water where the stone falls is set moving up and down, by the energy of the stone. This disturbance spreads out in circles, a wave, towards the edge of the pond. A leaf floating on the water just bobs up and down: it does not travel to the edge. The water does not flow to the edge either. Only the energy travels outwards: you can see it when the ripples make the grass at the edge move.

So a wave is a disturbance that carries energy from one place to another, without carrying the substance along.

Types of wave

Some waves need a substance to travel through; some don’t. The substance a wave travels through (air, water, a rope, the ground) is called its medium. Based on whether they need a medium, waves are of two types.

• A wave that needs a medium to travel is a mechanical wave. Water waves, a wave on a rope, and sound are mechanical waves. Sound can’t travel through empty space, where there is nothing to vibrate.
• A wave that can travel without any medium, even through empty space, is an electromagnetic wave. Light, X-rays and radio waves are electromagnetic waves. That is how the sun’s light reaches us across 150 million km of empty space.

Mechanical waves are of two types: transverse waves and longitudinal waves.

Transverse wave

A wave in which the particles of the medium vibrate (move to and fro) at right angles (perpendicular) to the direction in which the wave travels is called a transverse wave.

On the rope, the wave travels along the rope, but each bit of rope moves up and down, at right angles to it. That is a transverse wave. Ripples on water are treated as transverse waves too. Light and radio waves are transverse waves too, though they need no medium. A transverse wave has high parts, the crests, and low parts, the troughs. The crest is the part of the wave raised highest above the middle (rest) position, and the trough is the part pushed lowest below it.

Longitudinal wave

A wave in which the particles of the medium vibrate to and fro along the same direction in which the wave travels is called a longitudinal wave. Sound is a longitudinal wave.

When sound travels through air, the air molecules do the same as the rings of the slinky. A region where the air molecules are crowded closer together is a compression (C), and a region where they are farther apart is a rarefaction (R). The air molecules vibrate left and right, and the sound energy travels left to right, in the same direction. One compression and one rarefaction together make one complete wave of sound.

Some terms related to waves

These words describe a wave. Look at them on the rope wave in Figure 7.10, and in the wave machine below.

• Mean position: the middle line, where the particles of the medium rest when there is no wave.
• Crest: the part of a wave with the greatest upward displacement (movement) of the particles from their mean position: the top of a hump.
• Trough: the part of a wave with the greatest downward displacement of the particles from their mean position: the bottom of a dip.
• Amplitude: the greatest displacement of a vibrating particle from its mean position: the height of a crest above the middle line (or the depth of a trough below it). Its symbol is a (some books write A) and its SI unit is the meter (m). A bigger amplitude means more energy: a louder sound, or a taller water wave.
• Wavelength: the distance between two crests next to each other, or two troughs next to each other. Its symbol is the Greek letter λ (lambda) and its SI unit is the meter (m). In a sound wave, it is the distance from one compression to the next.
• Complete wave: one crest and one trough together (or one compression and one rarefaction) make one complete wave, also called one cycle. Its length is one wavelength.
• Frequency: the number of complete waves made (or complete vibrations) in one second. Its symbol is f and its SI unit is the hertz (Hz). A wave with 5 complete waves every second has a frequency of 5 Hz.
• Wave velocity: the distance a wave travels in one second. Its symbol is v and its SI unit is meters per second (m/s).

Each second, f complete waves go past, and each is one wavelength long. So in one second the wave travels f × λ meters:

$$\text{velocity } (v) = \text{wavelength } (\lambda) \times \text{frequency } (f)$$

Speed of sound in different media

Sound needs a medium, but it can travel through solids, liquids and gases. Its speed is different in each. It travels fastest in solids, slower in liquids, and slowest in gases, because the particles of a solid are packed close together and pass the vibration on quickly.

Medium	Speed of sound (roughly)
air (at 20 °C)	340 m/s
water	1,500 m/s
steel	5,000 m/s or more

What do we need to see the things around us? Can you see anything on a dark night without a torch or another source of light? No. We need light.

In the day, sunlight lets us see. At night we need other sources of light, such as an electric bulb, a candle, an oil lamp (diyo) or a torch. Things that give out their own light, such as the sun, a flame and a glowing bulb, are luminous.

Most things, like a book or a tree, give out no light of their own. We see them because light from a source falls on them and bounces back into our eyes. When light from an object, or light reflected from it, falls on the retina at the back of our eye, an image is formed there, and we see the object. In this section we look at how light bounces back from a surface: reflection of light.

When light falls on a mirror, it bounces back. In fact, light bounces back from the surface of every opaque object (an object light can’t pass through), smooth or rough.

Reflection of light

The bouncing back of light after it strikes a surface is called reflection of light. Light is reflected from smooth surfaces and from rough ones.

Regular reflection

When a beam of parallel rays of light falls on a smooth, flat surface, the reflected rays are also parallel to each other. This is called regular reflection. It happens at a mirror, a mobile screen, a polished metal plate and the surface of still water. Regular reflection forms an image: that is how we see our face in a mirror, or the mountains upside down in a still lake such as Phewa Tal.

Irregular reflection

When parallel rays fall on a rough or uneven surface, the reflected rays are scattered in different directions. This is called irregular reflection (or diffuse reflection). It happens at wood, the wall of a house, the leaves of plants, paper, cloth and almost everything around us.

Irregular reflection does not form an image, because the reflected rays go off in all directions. But this is exactly why it is useful: the scattered rays reach our eyes from wherever we stand. Irregular reflection lets us see the things around us.

Some terms related to reflection of light

Figure 7.15 shows a ray of light reflected by a plane (flat) mirror XY.

• Incident ray: a ray of light that comes from a source and strikes a surface. In the figure, IO is the incident ray. Its arrow shows that light goes from I to O.
• Point of incidence: the point where the incident ray strikes the surface: O.
• Reflected ray: the ray of light that bounces back after striking the surface. OR is the reflected ray; its arrow shows that light goes from O to R.
• Normal: a line drawn perpendicular (at 90°) to the surface at the point of incidence. ON is the normal; it makes 90° with the mirror XY.
• Angle of incidence: the angle between the incident ray and the normal: ∠ION, written ∠i.
• Angle of reflection: the angle between the reflected ray and the normal: ∠RON, written ∠r.

Laws of reflection of light

Regular reflection follows two rules, the laws of reflection:

1. The angle of incidence is always equal to the angle of reflection (∠i = ∠r).
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

The second law means that if the incident ray and the normal lie flat on your paper, the reflected ray lies flat on the paper too; it does not shoot up out of the page. That is why you could draw the whole of Activity 7.10 on one flat sheet.

Image formed in a plane mirror

Figure 7.17 shows how a plane mirror forms an image. Rays from the object strike the mirror and are reflected into the eye. To the eye, the reflected rays seem to come from a point behind the mirror. The eye sees the object there: that is the image. A drawing like this, showing rays and how an image forms, is called a ray diagram.

The image in a plane mirror is:

• virtual: the reflected light never really reaches the place behind the mirror where the image seems to be, so the image can’t be caught on a screen. (An image that light really reaches, and that can be caught on a screen, is a real image.)
• erect (upright, not upside down);
• the same size as the object;
• as far behind the mirror as the object is in front of it;
• laterally inverted: left and right are swapped. Raise your right hand in front of a mirror, and your image raises its left.

Some applications of reflection of light

• Microscope: a mirror under the stage of a microscope reflects light up through the slide, so that the tiny living things on it are bright enough to be seen.
• Mirrors on vehicles: the side and rear-view mirrors of buses, cars and motorbikes reflect the road behind, so the driver can see what is coming without turning round. (These are usually curved mirrors, which you will learn about in Grade 8.)
• At home: we use mirrors to comb our hair, shave, and to make a small room look bigger.
• Seeing everything around us, thanks to irregular reflection.

Periscope

A periscope is an instrument that uses the reflection of light to see over the top of something, from a hidden or low place. It is a tube with two plane mirrors, one at each end, each at 45°, facing each other and parallel. Light from the object enters the top opening and strikes the top mirror, which turns it straight down the tube. The bottom mirror turns it again, out of the bottom opening into your eye. So you can see things that are higher up, over a wall or a crowd, while you stay low.

Applications of the periscope

• a.A submarine uses a periscope to see the surface of the sea and the ships around it while it stays hidden under the water.
• b.In war, soldiers use periscopes to look out from a trench or over a wall, to watch the enemy without showing their heads.

7.1 Energy

1. Fill in the blanks

Use these words: lakhs, renewable, carbon dioxide, alternative, saving, non-renewable. (One word is not needed.)

2. Choose the correct answer

3. Differentiate between

4. Answer the following questions

5. Give reasons

7.2 Heat and temperature

1. Fill in the blanks and make a summary

Use these words: increase, 100 °C, warmth, temperature, decrease, 0 °C, kelvin, a thermometer, heat, 37 °C.

2. Choose the correct answer

3. Answer the following questions

4. Convert the following

5. Differentiate between

7.3 Wave

1. Fill in the blanks

Use these words: meter, frequency, medium, air, vibration, solid. (One word is not needed.)

2. Choose the correct answer

3. Differentiate between

4. Answer the following questions

5. Give reasons

7.4 Light

1. Fill in the blanks

Use these words: law, reflect, equal, parallel, perpendicular, refract. (One word is not needed.)

2. Choose the correct answer

3. Differentiate between

4. Answer the following questions